NIST Time Scale Data Archive

This page contains a table of leap seconds, the current difference between the UT1 and UTC time scales, and the current UT1 - UTC difference that is being broadcast by NIST (called the DUT1 correction).

The master clock pulses used by the WWV, WWVH, WWVB, ACTS and Internet Time Service (ITS) time code transmissions are referenced to the UTC(NIST) time scale. Occasionally, 1 s is added to the UTC time scale. This second is called a leap second. Its purpose is to keep the UTC time scale within ±0.9 s of the UT1 astronomical time scale, which changes slightly due to variations in the rotation of the Earth.

Coordinated Universal Time (UTC) is based on International Atomic Time (TAI), but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth. This correction keeps UTC in conjunction with the apparent position of the Sun and the stars, and it is the standard used for all general timekeeping applications.

The current difference between UTC and TAI is 35 seconds. (TAI is ahead of UTC by this amount)

The first leap second was inserted into the UTC time scale on June 30, 1972. Leap seconds are used to keep the difference between UT1 and UTC to within ±0.9 s. The table below lists all leap seconds that have already occurred, or are scheduled to occur.

All leap seconds listed in the table are positive leap seconds, which means an extra second is inserted into the UTC time scale. The sequence of events is:

23h 59m 59s - 23h 59m 60s - 00h 00m 00s

NOTE: A positive leap second will be added at the end of July 2015.

Leap Seconds Inserted into the UTC Time Scale

Date

MJD

Date

MJD

Date

MJD

Date

MJD

2012-06-30

56108

1998-12-31

51178

1989-12-31

47891

1979-12-31

44238

2008-12-31

54831

1997-06-30

50629

1987-12-31

47160

1978-12-31

43873

2005-12-31

53735

1995-12-31

50082

1985-06-30

46246

1977-12-31

43508

1994-06-30

49533

1983-06-30

45515

1976-12-31

43143

1993-06-30

49168

1982-06-30

45150

1975-12-31

42777

1992-06-30

48803

1981-06-30

44785

1974-12-31

42412

1990-12-31

48256

1973-12-31

42047

1972-12-31

41682

1972-06-30

41498

Current UT1-UTC values

This table lists the most recent differences between UT1 and UTC. This information is obtained from the United States Naval Observatory (USNO).

Leap seconds ensure that UT1 - UTC will always be held within ±0.9 s. The current value of UT1 - UTC is called the DUT1 correction and is obtained from the Bureau International des Poids et Mesures (BIPM). DUT1 corrections are broadcast by WWV, WWVH, WWVB and ACTS, and are printed below. These corrections may be added to received UTC time signals in order to obtain UT1.

The resolution of the DUT1 correction is 0.1 s, and represents an average value for an extended range of dates. Therefore, it will not agree exactly with the weekly UT1-UTC(NIST) values shown in the earlier table, which have 1 ms resolution and are updated weekly.